The invention relates to detection of anomalies related to a measurement of the hypnotic level of a subject. In order to detect when a measure indicative of the hypnotic level of a subject is anomalous either due to a medical reason or due to an interference, the measure is monitored and state information indicative of the activity of the autonomous nervous system of the subject and drug effect information indicative of the hypnotic drug effect in the subject are employed to check whether the measure fulfills a predetermined consistency condition requiring that the measure changes consistently with at least one of the drug effect information and the state information. An anomaly, indicative of an abnormal change in the measure, is detected when the measure fails to fulfill the predetermined consistency condition.
